Photolithographic techniques have wide use in fabrication of semiconductor devices, particularly in a fabrication of miniaturized electronic components. Photolithography processes comprise applying a photoresist composition to a substrate material, such as a silicon wafer. Once applied, the photoresist is fixed to the substrate to form a composite. The composite of substrate and photoresist is subjected to an image-wise exposure of radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the photoresist covered surface. Visible light, ultraviolet (UV) light, electron beam and x-ray radiant energy are energy sources in wide use in photolithography. After image-wise exposure, the photoresist coated substrate is treated with a developer solution to dissolve and to remove either the radiation-exposed areas or the unexposed areas of the photoresist.
One problem encountered when using photolithographic techniques, particularly in fabricating microcomponents, is back reflectivity or back reflection of light. Back reflection is a cause of thin film interference and reflective notching. Thin film interference results in changes in critical line width dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist film changes. Reflective notching occurs when the photoresist is patterned over substrates containing topographical features, which scatter light throughout the photoresist film, leading to line width variations. In an extreme case, reflective notching forms regions with complete resist loss.
Antireflective coatings such as DARC coatings and BARC coatings have been introduced into the photolithography process in order to reduce problems caused by back reflection. Antireflective coatings absorb radiation used for exposing the photoresist.
Substrate fabrication of semiconductor devices typically produces a number of surfaces comprised of dissimilar materials. Many of these surfaces do not have a uniform height, thereby rendering the wafer thickness non-uniform. For example, as is shown in prior art FIG. 1, the height of a material such as a boro-phosphosilicate glass (BPSG) layer 12 of the wafer section 10, does not have the same height at points 14, 16 and 18. Further, surfaces may have defects such as crystal lattice damage, scratches, roughness, or embedded particles of dirt or dust. For various fabrication processes that are performed, such as lithography and etching, unplanned non-uniformities in height and defects at the surface of the wafer or at the surface of any layer of the wafer must be reduced or eliminated.
One problem encountered with the DARC coating arises when a nitride layer or other type of layer is positioned in contact with the DARC coating. In particular, the problem arises when the layer overlays the DARC coating. It has been observed that the overlying layer, such as a silicon nitride layer, develops microparticles within the layer. The microparticles have a diameter of about 0.13 microns in the nitride film. The microparticles are formed as a result of one or more reactions between the DARC coating and the overlying layer. The microparticles are undesirable because they distort the topography of the silicon nitride surface or other capping surface as well as cause distortion at an interface with the DARC coating as is illustrated at 10 in prior art FIG. 1. The distortion occurs because the thickness of the cap, such as the silicon nitride cap, ranges from about 0.1-to- 0.2 microns, which is within the diameter range of the microparticles.
One approach to preventing or eliminating surface discontinuities includes taking action during fabrication in order to prevent surface height discontinuities from occurring in the first place. The Sasaki Patent, U.S. Pat. No. 5,226,930, which issued Jul. 13, 1993, describes a method for preventing agglomeration of colloidal silica in a silicon wafer. The patent describes forming an aqueous colloidal silicon dispersant from an aqueous sodium silicate solution and mixing a trialylhalosilane with the aqueous colloidal silicon dispersant to form trialkylsilane. With this treatment, colloidal silica is substantially prevented from undergoing gelation during drying. This chemical reaction then prevents a polishing slurry that includes colloidal silica from causing scratches or latent flaws on a wafer surface during polishing of the silicon wafers.